1. Field of the Invention
The present invention relates to circuits for the generation of ignition pulses for a lamp, such as for example a high-pressure gas discharge lamp, and to methods for the generation of ignition pulses for a lamp. Finally, the invention also relates to lamp ballasts, which have such ignition circuits.
2. Description of the Related Art
Generally it is the task of ignition circuits of the kind concerned to send so-called ignition voltage pulses to the lamp, which ensure a reliable lamp ignition during a certain phase condition of the mains voltage.
From WO 97/08921, the ignition circuit illustrated in FIG. 6 is known. The ignition circuit schematically shown in FIG. 6 has a choke 3, serving as a magnetic ballast, a pulse transformer 5, the secondary winding 6 of which is connected in series with the choke 3 and the high-pressure gas discharge lamp 4, and the primary winding 8 of which is connected in series with a switch element 9, and a pulse capacitor 7, wherein the pulse capacitor 7 on the one hand and the series circuit of the primary winding 8 and the switching element 9 on the other hand form a parallel circuit, which for its part is connected in series with a load resistance 13 to a controllable switch 10. The controllable switch 10 is for example a bipolar transistor or field effect transistor controlled in a rectifier bridge.
Further, there is present an auxiliary ignition capacitor 11 and a control circuit 12, which serves for control of the controllable switch 10. The control circuit 12 controls the controllable switch 10 temporally in dependence upon the appearance of an ignition pulse for the high-pressure discharge lamp 4, an ignition pulse being detected by means of an ignition pulse detector 15 which is connected with the pulse transformer 5 by means of a specific winding 14.
The functioning of the circuit shown in FIG. 6 is thereby as follows:
Initially, the controllable switch 10 is open, so that the parallel circuit formed of a pulse capacitor 7, the primary winding 8 of the pulse transformer 5 and the sidac 9 is separated from the a.c. voltage supply applied at the terminals 1. The control circuit, for example an ASIC, contains preferably a counter which is set in operation when a zero crossing of the mains voltage occurs or the mains voltage has reached a certain level, which corresponds to a certain switching angle. By counting down it can be determined when the required switching angle, i.e. the phase disposition required by the lamp manufacturers, between 60° EL to 90° EL or 240° EL to 270° EL, is attained. When the desired phase disposition is attained, the controllable switch 10 is closed, whereby the voltage applied at the auxiliary ignition capacitor 11 is reduced for a short time, since through the closing of the controllable switch 10 the pulse capacitor 7 is connected in parallel with the auxiliary ignition capacitor 11. The secondary winding 6 of the pulse transformer 5 is itself of low resistance.
After the closing of the controllable switch 10, normal ignition behavior arises, i.e. the voltage applied at the pulse capacitor 7 increases through the charging of the pulse capacitor 7 via the load resistance 13, so that the voltage applied to the lamp 4 or the auxiliary ignition capacitor 11 also increases. When the switching voltage of the sidac 9 is attained, this short-circuits and the pulse capacitor is discharged via the primary winding 8 of the pulse transformer 5 and the sidac 9, through which an ignition pulse is generated at the high-pressure discharge lamp 4, which is reported to the control circuit 12 via the coupled winding 14 and the ignition pulse detector 15.
With detection of an ignition pulse, the control circuit 12 immediately opens the controllable switch 10, so that the oscillation circuit formed of the pulse capacitor 7, the sidac 9 and the primary winding 18 of the pulse transformer 5 very quickly decays, since no new energy is delivered to this oscillation circuit. Through this, the holding current of the sidac 9 is very quickly undershot, which allows the switch 10 to be again closed, shortly after the opening of the switch 10, so that a rapid ignition pulse sequence can be attained.
A disadvantage of this circuit is that it does not take into account that the ignition voltage sinks with the line capacitance.
From EP 479351 A1 there is known a self-adapting ignition circuit, which attempts to provide assistance with regard to this problem.
In accordance with this publication, there are provided two pulse capacitors which can be switched parallel to one another. If a circuit (IV in FIG. 1) now detects that the ignition pulse applied at the lamp itself does not have sufficient amplitude, the second pulse capacitor is switched in parallel with the actual first pulse capacitor by means of actuation of a switch, which as is known increases the capacitance, through which in a following ignition process the ignition pulse amplitude is correspondingly increased.
The procedure in accordance with EP 479 351 A1 is thus such that one begins always with an ignition procedure with the employment of a single pulse capacitor, and for the event that the amplitude of the ignition pulse at the lamp is not sufficient, a second capacitor is switched in parallel. There is thus provided a discrete increase of the capacitance and thus of the ignition pulse amplitude. A reduction of the capacitance is, in contrast, not provided for.